Determination of iodine detectability in different types of multiple-energy images for a photon-counting detector computed tomography system

In addition to low-energy-threshold images (TLIs), photon-counting detector (PCD) computed tomography (CT) can generate virtual monoenergetic images (VMIs) and iodine maps. Our study sought to determine the image type that maximizes iodine detectability. Adult abdominal phantoms with iodine inserts of various concentrations and lesion sizes were scanned on a PCD-CT system. TLIs, VMIs at 50 keV, and iodine maps were generated, and iodine contrast-to-noise ratio (CNR) was measured. A channelized Hotelling observer was used to determine the area under the receiver-operating-characteristic curve (AUC) for iodine detectability. Iodine map CNR (0.57 ± 0.42) was significantly higher (P < 0.05) than for TLIs (0.46 ± 0.26) and lower (P < 0.001) than for VMIs at 50 keV (0.74 ± 0.33) for 0.5 mgI/cc and a 35-cm phantom. For the same condition and an 8-mm lesion, iodine detectability from iodine maps (AUC = 0.95 ± 0.01) was significantly lower (P < 0.001) than both TLIs (AUC = 0.99 ± 0.00) and VMIs (AUC = 0.99 ± 0.01). VMIs at 50 keV had similar detectability to TLIs and both outperformed iodine maps. The lowest detectable iodine concentration was 0.5 mgI/cc for an 8-mm lesion and 1.0 mgI/cc for a 4-mm lesion.